Vehicle number plates (i.e. license plates) are commonly made to be retroreflective in order to enhance the visibility of the number plate at night.
Certain methods of fabricating retroreflective number plates commonly involve the lamination of a thin retroreflective film (also commonly referred to as a retroreflective sheeting) to the back-side of a thick, clear plastic plate. The lamination of the retroreflective film to the clear plastic plate is often achieved with an optically clear pressure-sensitive adhesive. The clear plastic plate is commonly made from either acrylic or polycarbonate resin and is typically about 0.125 inches thick.
When retroreflective number plates are used for the purposes of vehicle registration, the number plates applied to each individual vehicle must contain a unique set of alphanumeric characters or other characters or symbols. These characters are printed (or otherwise created) on either the back surface of the plastic plate or on the top surface of the retroreflective film prior to laminating the retroreflective film to the plastic plate. The most common method currently employed to print these characters is to use a computer-controlled printer such as a thermal transfer printer, an ink jet printer, or a laser printer to print the characters on the front-surface of the retroreflective sheeting.
The retroreflective film employed to fabricate a number plate can include any of commonly known retroreflective sheeting constructions and can incorporate any of the commonly known retroreflective elements including glass microspheres or microprisms. The three most common retroreflective sheeting constructions include enclosed lens retroreflective sheeting (incorporating the glass microspheres), encapsulated lens retroreflective sheeting (also incorporating glass microspheres) and microprismatic retroreflective sheeting. U.S. Pat. Nos. 2,407,680 and 4,367,920 provide detailed descriptions of the design and manufacture of enclosed lens sheeting and are incorporated herein by reference. Also, U.S. Pat. Nos. 3,109,178 and 4,025,159 provide detailed descriptions of encapsulated lens sheeting, and U.S. Pat. Nos. 3,689,346 and 4,588,258 provide descriptions of microprismatic sheeting, which are incorporated herein by reference.
Not all retroreflective sheeting materials, however, can be successfully printed through thermal transfer printers, laser printers, ink jet printers, or other printers. To be acceptable for use in the fabrication of numbered plates, a high print quality is required. Generally, the print quality is judged across several different criteria. First, the coverage of the print must be complete without any signs of pin-holes or other print voids, and the depth of coverage should be full enough to provide a deep, fully saturated print. When printing opaque colors such as black, the surface of the retroreflective sheeting should not show through the print. Second, the edges of printed alphanumeric characters must be straight, clean and sharp. Corners should be square. Print edges that are wavy or undefined are generally not acceptable. Third, the printed surface should be uniform without any smudging or smearing. This smearing or smudging effect is especially common with thermal transfer printing when printing letters such as “E”, “F”, “H”, or “T” from left-to-right where there exists a long horizontal bar of printing.
Obtaining a high quality print through a laser printer presents several additional challenges. In many instances, materials which successfully print through a thermal transfer printer or ink jet printer may not yield the same quality results through a laser printer. Generally speaking, laser printers operate through several basic steps. First, a photoreceptive rotating drum inside a laser printer is given an overall positive electrostatic charge. Second, through the use of a laser, this photoreceptive rotating drum is then given a negative electrostatic charge in selective areas corresponding to the image to be printed. Third, the rotating drum is brought into contact with the printing toner, which is a very fine colored plastic powder that has been positively charged. Due to the electrostatic attraction between the negatively charged areas of the rotating drum and the positively charged toner, the toner particles will bond to these selective areas of the drum. On the other hand, the toner particles will not bond to those areas of the rotating drum still retaining a positive charge, which corresponds to the non-printed background on an image.
Next, the printing substrate (i.e the retroreflective sheeting) is passed through the laser printer and is given its own negative electrostatic charge. The magnitude of the negative electrostatic charge on the printing substrate is greater than the negative charge on the rotating drum. The printing substrate is next brought into contact with the rotating drum, and the toner particles are transferred from the drum to the printing substrate through electrostatic attraction. To prevent the printing substrate from clinging to the positive areas of the rotating drum, the electrostatic charge on the substrate is immediately discharged after the toner is transferred to it. Next, the printing substrate is passed through a set of heating rollers to fuse (i.e. melt) the toner to the substrate. Finally, the substrate passes through a cleaning mechanism to remove any residual toner particles.
Laser printers create an especially difficult printing challenge for retroreflective sheeting. First, the sheeting must be able to withstand the high temperatures associated with the toner fusion process. This high heat can cause the retroreflective sheeting to wrinkle or become dimensionally distorted, which can lead to a poor print quality or make subsequent lamination to a plastic plate very difficult. Further, when printing over larger areas with a laser printer, the printed image should remain consistently bold in appearance across the entire printed area without any fading or lightening of the image. Additionally, each individual alphanumeric character or symbol to be printed should only appear once without any secondary repeated images of the same character or symbol. When such repeated imaging print defects occur, they often appear as a lighter “shadow” or “ghosting” of the initially printed character. In many instances, the occurrence of such secondary repeated images will then impact the printing of any subsequent characters as well. It is believed that such secondary repeated images or shadow effects are a result of the toner not completely transferring to or bonding to the retroreflective sheeting surface. These latter two performance attributes are especially important when printing the larger sizes of many European vehicle number plates, which may exceed 20 inches in length.
In today's environment of rapidly changing technology, it is very common for multiple printer technologies to be located in the same number plate manufacturing facility. For example, there may be several different models of both thermal transfer printers and laser printers located in the same facility. Furthermore, different models and different brands of printers may function very differently. For example, the chemistry of ink jet printing inks utilized in different printer brands can vary greatly and have different drying or printing characteristics. Likewise, the toner utilized in different brands of laser printers may fuse at different operating temperatures. With the desire to maintain low inventory levels and to simplify manufacturing supply chains, there is an additional need that the same retroreflective sheeting be capable of producing high quality prints through multiple printer technologies and printer models. It would be advantageous for the same retroreflective sheeting to be printable on multiple models and brands of thermal transfer, laser, and ink jet printers.
Certain conventional methods have attempted to overcome these printing challenges by creating a matte surface on the face of the enclosed lens sheeting during conventional manufacturing processes. Though these conventional approaches have allowed for an improved print quality, they do not always provide consistent print results, and there is a significant potential for scrap using this method of manufacturing. Moreover, the most pronounced problem with this approach is the presence of pin-holes or printing voids when thermal transfer printing on the surface of the enclosed lens sheeting. These defects are often caused by surface imperfections in the top-coat of the retroreflective film, including air-bubbles, surface pits, pin-holes, and similar coating defects. Further, many such sheetings cannot withstand the high temperatures associated with laser printers or do not provide the proper surface to allow fusion of the toner. Such sheetings often display a significant amount of dimensional distortion or may even melt when being processed through a laser printer. Further, the overall print quality through laser printers may be very poor with a significant amount of print defects.